The present process relates to a field of waste processing, and it can mainly be used to produce a gasoline, diesel and fuel oils of waste plastics by pyrolysis and catalytic cracking.
Pyrolysis is the known process of thermal destruction of hydrocarbons in oxygen-free environment under temperature of 400-900xc2x0 C. and small excess pressure. This process, for example, is widely used in petroleum refinery for obtaining low molecular monomers from naphtha, and it can used for waste plastics processing with fuels production as an alternative of its incineration or landfilling.
A number of operating condition variables affects the thermal destruction. These are so-called 3 T""s: temperature, time (residence time) and turbulence (or mixing). It is possible to understand the turbulence as method of gas/solid (feedstock) contacting and conditions of mixing pyrolysis products together with gas. A degree of reduction in size has an essential effect. High temperature (700-900xc2x0 C.) and short residence time (1 second and less) are used for obtaining great quantity of low molecular monomers from petroleum feedstock. It is an endothermic process demanding heat supply from outside.
At present it is known a method of waste plastics pyrolysis in a fluidized bed (Patents: JP.52155603A2, U.S. Pat. No. 3,901,951, EP0502618, U.S. Pat. No. 5,364,995, EP0567292 and U.S. Pat. No. 5,481,052; investigations of Prof. W. Kaminsky in Hamburg University). Grainy inert materials, for example, quartz, sand or ceramic crumb, are used for making a fluidized bed. This material can be used as a circulating heat carrier, being heated in a separate apparatus. A fluidizing agent is inert gas or circulating pyrolysis gas. Preliminary shredding a feedstock is necessary for this technology. A reactor is provided with equipment for a feedstock supply and withdrawing a possible solid residue. A gas stream (pyrolysis products and fluidizing gas) and particles of inert material, escaping from the location of a fluidized bed, are separated within a cyclone.
In comparison with other methods of waste plastics processing, for example, those carried out in the rotary kiln reactor and the shaft reactor, pyrolysis in a fluidized bed has the following advantages: design simplicity, compactness, no moving parts (for the kiln reactor), low operating cost and lower capital cost, the increase of products yields, the enhance of products quality. These advantages are associated with well-known properties of a fluidized bed: uniformity of temperature field without temperature gradients, (those gradients are typical for above-mentioned packed bed reactors), effective mass exchange and a possibility to use a circulating solid heat carrier.
However, fluidization technique has also its own disadvantages, when it is used for such chemical processes as pyrolysis requiring very short residence time. Among these disadvantages are:
Mixing feedstock in the whole volume of a fluidized bed,
Impossibility to ensure short contact time,
Back mixing of pyrolysis products.
A possibility of very short residence time is provided by pyrolysis carried out in an upflow tubular reactors (risers), in which circulating fine solid heat carrier is transported in delute phase with inert gas or circulating pyrolysis gas (Patents: U.S. Pat. Nos. 4,147,593, 5,136,117 and 5,792,340). Feedstock, heat carrier particles and transporting gas are delivered to the bottom of the riser. Pyrolysis products, fluidizing gas and circulating inert material particles are separated in a cyclone. This system is also named as a circulating fluidized bed. Risers enable essentially to reduce residence time in comparison with a fluidized bed. For example, transition from catalytic cracking of petroleum feedstock in a fluidized bed to catalytic cracking in a riser enable to reduce catalyst residence time from several minutes to several seconds. Also, conditions of feedstock/solid contacting get essentially better in a system, employing a riser.
However, conversion in a riser has its own disadvantage. A vector of gas velocity and a vector of particle gravity are directed to opposite directions. It creates the known xe2x80x9cslip effectxe2x80x9d, when particle velocity relative to reactor walls falls behind gas velocity by terminal velocity. This circumstance reduces solid particles residence time in the reactor, and it promotes back mixing of solid particles and, respectively, of gas, which is observed at the reactor walls. Although, this mixing is considerably inferior in comparison with that in a fluidized bed.
This disadvantage is absent in a downflow tubular reactor, known under names: xe2x80x9cdownerxe2x80x9d, xe2x80x9cdownflow circulating fluidized bedxe2x80x9d, xe2x80x9cdowncomerxe2x80x9d). In this reactor the above-mentioned vectors coincide. The main advantages of this type of configuration over riser reactors are a short residence time with a narrow residence time distribution, little or no solids back mixing, and lower pressure drops since gravity acts in the same direction as the flow stream. Feedstock and heat carrier particles are delivered to the top of the reactor from a packed or fluidized bed placed in a special section or a separate apparatus. After going out of the downer, Products and particles of a circulating solid heat carrier are separated in a cyclone. A method and a reactor are patented in connection with the fluid catalytic cracking process (FCC) (Patents: U.S. Pat. Nos. 4,385,985, 5,449,496, 5,843,377, 5,582,712). The expediency of using these reactors for ultrapyrolysis of organic feed is discussed in scientific papers.
However, the use of risers and downers for pyrolysis of mixed feed, components of which have various conversion velocity, or for pyrolysis of shredded feedstock with a large range of particles size, has a restriction since hard-converted or coarse particles can break through a reactor. Or, in this case, it is necessary to select a reaction volume for these components. First of all, it concerns the mixed plastics, composing the larger part of municipal solid waste and containing mainly of polyethylene, polypropylene, polyvinyl chloride and polystyrene. In this respect, a fluidized bed reactor enables to process a feedstock with wider range of conversion velocity distribution.
There are known attempts to solve this problem by step-by-step increasing waste plastics pyrolysis temperature, when lower temperature is established for pyrolysis of light-converted polymers. Then, this temperature is increased for pyrolysis of hard-converted polymers (Patents: U.S. Pat. Nos. 5,386,070, 5,895,827). increasing temperature on a next stage of pyrolysis can be achieved also with a delivery of a hot circulating solid heat carrier, as it is widely used for the FCC process including catalytic cracking in a downer (Patent: U.S. Pat. No. 4,514,285).
Potential presence of polyvinyl chloride is one more peculiarity of mixed plastics pyrolysis. Hydrogen chloride, effecting equipment corrosion, is evolved during such plastics pyrolysis. Two-stage processing is used in this case. At first, feed melting is carried out at temperature of 200-300xc2x0 C. The evolved hydrogen chloride is delivered to an adsorber with calcium oxide or similar compound. Melted feed is subjected to a following pyrolysis (Patents: U.S. Pat. Nos. 3,901,951, 5, 821,395).
During pyrolysis of mixed plastics waste at respectively low temperature (480-590xc2x0 C.), it is possible to yield about 85% of liquid hydrocarbons (C5-C40). These hydrocarbons are a good feed for following catalytic cracking since they do not consist of sulfur, basic nitrogen and such metals poisoning a catalyst as nickel and vanadium.
Methods of pyrolysis, described in the patents U.S. Pat. Nos. 5,481,052 and 5,821,395 of BP Chemicals, Ltd., England and the patent U.S. Pat. No. 5,976,355 of Stone and Webster Engineering Corp., USA are the nearest methods to the presented method. In the patents of BP Chemicals, Ltd., waste plastics pyrolysis is carried out in a fluidized bed. In the patent of Stone and Webster Engineering Corp., thermocatalytic conversion of hydrocarbon feedstock (catalytic cracking of oil feedstock) is carried out in a downflow tubular reactor named as a downflow riser reactor.
A general feature of the above-mentioned patents of BP Chemicals is the fact that pyrolysis of shredded or preliminary melted waste plastics is carried out in contact with grainy inert material, for example with sand, fluidized by oxidant-free gas including circulating pyrolysis gas. The process is conducted under temperature of 300-600xc2x0 C. and under pressure. Solid particles, entrained from a fluidized bed, are caught in a cyclone and/or in a guard bed. According to U.S. Pat. No. 5,481,052, required heat is delivered in the reactor by any fluidizing gas. According to U.S. Pat. No. 5,821,395, this heat delivers by burnt pyrolysis gas by means of a tube heat exchanger placed inside of the fluidized bed. The patent U.S. Pat. No. 5,821,395 is intended, first, for pyrolysis of waste plastics containing chlorinated polymers, for example, polyvinyl chloride (PVC). The patent provides recovery of the hydrogen chloride in a packed bed adsorber of a proper grainy adsorbent, for example calcium oxide.
However, the above-discussed methods of waste plastics pyrolysis in a fluidized bed according to the patents U.S. Pat. Nos. 5,481,052 and 5,821,395 of BP Chemicals have the following disadvantages inherent in fluidized bed processes:
1. Impossibility to ensure short contacting time,
2. Back mixing pyrolysis products,
3. Nonuniformity of the fluidized bed, which is expressed in a presence of gas bubbles and particles clusters.
Pyrolysis of light converted feed requires short residence time up to milliseconds as it is accompanied by secondary reactions of condensation reducing a yield of valuable olefins and, respectively, increasing the yield of less valuable aromatic and high molecular hydrocarbons. Nonuniformity of a fluidized bed worsens the conditions of feed/catalyst particles contacting at catalytic cracking.
Besides, the methods according to the patents U.S. Pat. Nos. 5,481,052 and 5,821,395 of BP Chemicals have additional disadvantages:
1. Delivery of heat into a reactor is inefficient, and it has serious disadvantages in operation. For example, using an inner heat exchanger according to the methods of the patent U.S. Pat. No. 5,821,395 increases some more fluidized bed nonuniformity, and it is accompanied by heat exchanger erosion.
2. There is not possible to control feed residence time at its constant flow rate that reduces a technological flexibility of the process.
A method of hydrocarbon feed conversion in a downflow tubular reactor according to the patent U.S. Pat. No. 5,976,355 is deprived of these disadvantages but it has own disadvantages:
1. In case a use of polymers mixture with a different conversion rate, for example shredded plastics from municipal solid waste, hard converted or coarse particles of feedstock can xe2x80x9cfall throughxe2x80x9d a reactor. The reaction volume should be selected as applied to these components but then pyrolysis of light converted particles will carry out in not optimal regime.
2. Respectively, there are not technological methods of a control of hard converted feed conversion degree.
3. Since a method of petroleum feed catalytic cracking does not foresee solid feed processing, such process stages as feed melting, separating and adsorption of hydrogen chloride are absent.
Therefore despite existence of various processes for pyrolysis there strongly felt a need in a two-stage process of waste plastics pyrolysis, ensuring elimination of the above-mentioned disadvantages of the known in the art processes.
The raised task is solved by the way that waste plastics preliminary shredded are delivered to the top of a downflow tubular reactor (downer). A hot circulating grainy inert heat carrier is also supplied into this downer from a feeder-fluidized bed apparatus, which is locate above the downer. The carrier is heated in a separate fluidized bed apparatus-combustor and is supplied in the said feeder-apparatus by a transport line. The shredded waste plastics and some quantity of inert gas entrained from the fluidized bed of the feeder are mixed with the heat carrier causing melting and pyrolysis of waste plastics in a downflow stream. Pyrolysis proseeds at a temperature between 300.degree. C. and 600.degree.C. and formed hydrocarbons residence time of 0.5 to 3 seconds producing hydrocarbon oils (the 1st stage of the process).
The mixture of the heat carrier, pyrolysis products and the entrained inert gas enter in an upper part (freeboard) of a fluidized bed reactor placed below, where gravitational separating the solid and gaseous phases is carried out. The heat carrier and unconverted waste plastics fall down in the fluidized bed, where pyrolysis is completed (the 2nd stage of the process).
The products of this pyrolysis stage together with the fluidizing inert gas and the entrained heat carrier particles enter the reactor freeboard and are mixed with the 1st stage products. A combined stream of the pyrolysis products, the inert gas and the entrained heat carrier particles are subjected by additional separating in a cyclone. After that the pyrolysis products are cooled by a cooled liquid pyrolysis residue (quenching) in a transfer line and supplied in a fractionating column for final cooling and fractionating pyrolysis products.
The heat carrier is stripped by inert gas from entrained hydrocarbons in a separate fluidized bed apparatus. The stripped hydrocarbons are returned into the freeboard of the fluidized bed reactor, and the heat carrier is transported to a fluidized bed combustor, where at temperature between 600.degree.C. and 900.degree.C. it is carried out combustion of coke formed during pyrolysis. Then, the heat carrier is separated from flue gas and returned to the feeder-apparatus.
In addition to controlling the process technological regime and feed conversion degree by virtue of a change of a feed rate, temperature and heat carrier circulation rate, the following additional technological measures are employed in the present process:
a) a control of residence time in the tubular reactor by a change of a rate of inert gas entrained from the feeder; this measure is achieved by means of a control of a pressure drop between the feeder and the fluidized bed reactor,
b) a change of bed height in this reactor,
c) a control of temperature in this reactor by an additional stream of the hot teat carrier from the feeder-apparatus.
In addition, a possibility to supply the shredded waste plastics directly in the fluidized bed of the feeder-apparatus is foreseen for preliminary mixing with the heat carrier and melting. In this case, temperature in the fluidized bed reactor is controlled by an additional stream of the hot heat carrier supplied from the combustor via a separate transport line.
If liquid waste plastics, preliminary melted in a separate apparatus, are supplied in the tubular reactor, pyrolysis can be carried out only in the tubular reactor and be over in the freeboard of a fluidized bed apparatus placed below, where, at the same time, separating and stripping the heat carrier are carried out. If chlorinated plastics (for example polyvinyl chloride) is processed, a generated hydrogen chloride is recovered by a proper adsorbent in a separate apparatus.
Obtained liquid pyrolysis products can be used as a high quality feedstock for a fluid catalytic cracking unit.